Dumbbell



FIG. 1 is a front, top and right side perspective view of a dumbbell, showing my new design;

FIG. 2 is an exploded front, top and right side perspective view thereof;

FIG. 3 is a front view thereof;

FIG. 4 is a rear view thereof;

FIG. 5 is a left side view thereof;

FIG. 6 is a right side view thereof;

FIG. 7 is a top view thereof; and,

FIG. 8 is a bottom view thereof.

The broken lines in the drawings illustrate the portions of the dumbbell, which form no part of the claimed design. 

CLAIM The ornamental design for a dumbbell, as shown and described. 